Most modern electronic devices manufactured today contain at least one electrical signal line which is an unwanted source of electrical xe2x80x9cnoisexe2x80x9d, thereby adversely affecting other electronic circuits, both within and external to the electronic device. Generally speaking, this noise exists in the form of electromagnetic interference (EMI) of nearby electrical signals by the offending electrical signal. This EMI may be conducted from the offending electrical signal line to others by way of an electrically conductive path. Alternately, the interference may be radiated from the offending electrical signal line to nearby circuits without the benefit of a directly conductive connection. Oftentimes, the result of such radiated or conducted noise is erroneous or improper operation of the circuit being affected by the EMI, due primarily to unexpected voltage changes in the affected circuit. As a result, protecting electrical circuits from EMI that is generated by other signal lines has long been an important facet of the electronic circuit and device design process.
One example of a source of such noise is a voltage inverter, such as a Royer DC/AC (direct-current/alternating-current) inverter from the prior art designed to convert a DC power source into an AC power source. Voltage inverters are used in many different electronic applications. Specifically with respect to laptop computers and other electronic instruments employing flat panel displays, voltage inverters are often utilized as the power source for the backlight required for those displays. Due to the switching nature of voltage inverters and the large relative current levels typically involved, significant amounts of EMI are both radiated and conducted into surrounding circuitry located within the device utilizing the voltage inverter. The power spectral density of this EMI typically takes the form of noise spikes at the fundamental frequency and harmonic frequencies at which the voltage inverter oscillates.
Several methods of protecting circuits from EMI generated by voltage inverters have been employed previously. Many such methods involve protecting the sensitive circuits of the electronic device involved from the noise effects of the inverter. For example, the electronic circuit designer often attempts to structure the physical layout of the electronic circuits on a printed circuit board (PCB) so that the generated EMI of the inverter will have an attenuated effect on other surrounding circuits. Such efforts include physically routing any offending signals remotely from other sensitive signal lines and circuits, utilizing additional ground planes within the PCB to electrically shield and separate the voltage inverter from surrounding circuits, and the like. Unfortunately, such efforts normally require exorbitant amounts of a PCB designer""s time and effort, and are also error-prone, requiring multiple circuit design revisions in order to reduce sufficiently the effects of the noise on the device.
Other similar solutions involve more substantive circuit additions to shield radiated and conducted noise from circuits that are sensitive to that noise. These additions include the use of large and complex filters on the PCB, chokes, additional metal shielding, shielded cables, and so on.
In contrast to the solutions above, more recent approaches to the problem involve changing the nature of the offending circuit to make the oscillating signal involved less of a noise source to surrounding circuitry. For example, one proposed solution has been to xe2x80x9cditherxe2x80x9d the switching frequency of the inverter by adding a small noise signal to the signal responsible for the oscillation. Dithering of this signal results in displacing the frequency spectrum of the offending noise a small amount, but does not lower the power level of the frequency spectrum. This solution has been utilized in devices in which other circuits within the device are sensitive to noise at particular frequencies, because the small displacement in the frequency spectrum of the oscillating signal may aid in reducing the effects of the noise on that circuit. However, many electronic devices are susceptible to noise across a wide range of frequencies, making this solution inapplicable in such cases. For example, dithering of the oscillating signal is particularly ineffective for electronic devices such as electronic test and measurement instruments, which often are employed to investigate electronic signals over a very wide band of the frequency spectrum.
Other prior art solutions, such as those indicated in xe2x80x9cCurrent control technique for improving EMC in power converters,xe2x80x9d ELECTRONIC LETTERS, Vol. 37, No. 5, pp. 274-275 (Mar. 1, 2001) by Giral et al., and xe2x80x9cImprovement of power supply EMC by chaos,xe2x80x9d ELECTRONIC LETTERS, Vol. 32, No 12, p. 1045 (Jun. 6, 1996) by Deane et al., focus on the use of chaotic control of DC/DC power converters to reduce the electromagnetic interference normally generated by such circuits. Such solutions succeed in reducing the peaks of the frequency spectrum due to the control signal associated with such converters by spreading out the power of the spectrum at the fundamental and harmonic frequencies. However, such solutions typically do not ensure failsafe operation of the converter being driven by the offending control signal due to its chaotic nature. Adding chaotic control as described by the prior art does not guarantee that the switch will not remain in the closed position, thus potentially causing permanent damage to the inductor of the converter by way of sustained electrical current. By the same token, the circuit described may not prevent excessive periods of time during which the inductor is not being charged, thus allowing the output voltage of the power supply to drop unacceptably.
Another solution, identified by Cahill in U.S. Pat. No. 5,263,055, entitled xe2x80x9cAPPARATUS AND METHOD FOR REDUCING HARMONIC INTERFERENCE GENERERATED BY A CLOCK SIGNALxe2x80x9d, implements a periodic clock signal that is frequency modulated, or alternately, phase modulated, by the output of a pseudorandom noise signal generator. While the power spectral energy of the fundamental and harmonic frequencies of the periodic clock signal is reduced, no control mechanism is present which ensures that the changing frequency of the modulated signal remains within the limits required of the circuit that is being driven by that signal. Hence, such a method, as applied to a DC/AC voltage inverter, is also likely to allow the inverter to remain in a nonswitching state for lengthy periods of time occasionally.
From the foregoing, despite previous attempts to mitigate or reduce EMI generated by DC/AC voltage inverters, a need still exists for a reliable method of reducing the EMI generated by such inverters. Such a method should both reduce the EMI generated while ensuring that the timing characteristics of the control signal driving the voltage inverter reside within a specified range to ensure effective, nondestructive operation of the inverter and its load.
Embodiments of the invention, to be discussed in detail below, provide a switching control circuit for a voltage inverter. A randomized signal generator is employed to create a randomized signal used as input for a frequency range converter. This range converter, in turn, produces a frequency modulation signal, the current state of which is based on the current state of the randomized signal. Additionally, the frequency range converter limits the current state of the frequency modulation signal so that the oscillating signal that is ultimately produced will operate within the specified frequency range. A variable frequency oscillator then generates the oscillating signal, the frequency of which is based on the current state of a frequency modulation signal. A logic inverter then inverts the oscillating signal. Both the oscillating signal and the inverted oscillating signal are then employed to drive the voltage inverter.
By modulating the frequency of the oscillating signal in this manner, the overall EMI produced by the operation of the voltage inverter is reduced in comparison to those voltage inverters that employ oscillating signals of a fixed frequency. Furthermore, by restricting the frequency of the oscillating signal to the specified frequency range, the proper operation of the voltage inverter driven by the oscillating signal and its inverted counterpart is maintained, thus helping to prevent unacceptable voltage dropouts and irreparable damage to the inverter or its load.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.